There are two primary mechanisms of erosion. The first is erosion caused by direct impingement. Normally, the most severe erosion occurs at fittings that redirect the flow such as at elbows and tees. The particles in the fluid can possess sufficient momentum to traverse the fluid streamlines and impinge the pipe wall. The other mechanism is erosion caused by random impingement.
This type of erosion occurs in the straight sections of pipe even though there is no mean velocity component directing flow toward the wall. However, turbulent fluctuations in the flow can provide the particles with momentum in the radial direction to force them into the pipe wall, but the turbulent fluctuations are a random process, therefore, it is termed random impingement. These two mechanisms can cause different types of erosion based on the fluid compositions, velocity, and configurations of piping systems. Venkatesh provides a good overview of erosion damage in oil wells. Regardless of the erosion mechanism, the most vulnerable parts of production systems tend to be components in which:
- The flow direction changes suddenly.
- High flow velocities occur that are caused by high volumetric flow rates.
- High flow velocities occur that are caused by flow restrictions.
Components and pipe systems upstream of the primary separators carry multiphase mixtures of gas, liquid, and particulates and are consequently more likely to suffer from particulate erosion, erosion-corrosion, and droplet erosion. Also the vulnerability of particular components to erosion heavily depends on their design and operational conditions. However, the following list is suggested as a rough guide to identify which components are most vulnerable to erosion (the first on the list being most likely to erode):
- Sudden constrictions;
- Partially closed valves, check valves, and valves that are not full bore;
- Standard radius elbows;
- Weld intrusions and pipe bore mismatches at flanges;
- Long radius elbows, miter elbows;
- Blind tees;
- Straight pipes.
The rate of sand erosion is determined by following factors :
- Flow rate of sand and the transport manner;
- Velocity, viscosity, and density of the flowing fluid;
- Size, shape, and hardness of the particles (sand);
- Configuration of the flow path such as straight tubing, elbow, or tee.
The effect of configuration of the flow path has already be described in the last section (section 18.2), only the effects of these factors are detailed in following programs.
Flow Rate of Sand and Manner of Transport
The nature of sand and the way in which it is produced and transported determines the rate of erosion within a production system. The sand production rate of a well is determined by a complex combination of geological factors, and it can be estimated by various techniques. Normally, new wells produce a large amount of sand as they “clean up.” Sand production then stabilizes at a relatively low level before increasing again as the well ages and the reservoir formation deteriorates. The sand production rate is not stable, and if a well produces less than 5 to 10 lb/day, it is often regarded as being sand free. However, this does not eliminate the possibility that erosion may be taking place.
The sand transport mechanism is an important factor for controlling sand erosion. Gas systems generally run at high velocities (>10 m/s), making them more apt to erosion than liquid systems. However, in wet gas systems sand particles can be trapped and carried in the liquid phase. Slugging in particular can periodically generate high velocities that may significantly enhance the erosion rate. If the flow is unsteady or the operational conditions change, sand may accumulate at times of low flow and be washed out through the system when high flows occur. The flow mechanisms may act to concentrate sand, increasing erosion rates in particular parts of the production system.
Velocity, Viscosity, and Density of the Fluid
The sand erosion rate is highly dependent on, and proportional to, the sand impact velocity. When the fluid velocity is high enough, the sand impact velocity will be close to the fluid velocity, and erosion will be an issue. Therefore, erosion is likely to be the worst where the fluid flow velocity is the highest. Small increases in fluid velocity can cause substantial increases in the erosion rate when these conditions are satisfied. In dense viscous fluids particles tend to be carried around obstructions by the flow rather than impacting on them. While in low-viscosity, low-density fluids, particles tend to travel in straight lines, impacting with the walls when the flow direction changes. Sand erosion is therefore more likely to occur in gas flows, because gas has a low viscosity and density, and gas systems operate at higher velocities.
Sand Shape, Size, and Hardness
Sand sizes in hydrocarbon production flow depend on the reservoir geology, the size of the sand screens in the well, and the breakup of particles as they travel from the reservoir to the surface. Without sand exclusion measures, such as downhole sand screens, particle sizes typically range between 50 and 500 microns.With sand exclusion in place particles larger than 100 microns are usually excluded. A sand particle density of about 2600 kg/m3 is generally accepted.Particle size influences erosion primarily by determining how many particles impact on a surface.
Erosion often causes localized grooves, pits, or other distinctive patterns in the locations of elevated velocity. Corrosion is usually more dispersed and identifiable by the scale or rust it generates. Erosion-corrosion is a combined effect of particulate erosion and corrosion. The progression of the erosioncorrosion process depends on the balance between the erosion and corrosion processes. Erosion-corrosion can be avoided by ensuring that operating conditions do not allow either erosion or corrosion.
In a purely corrosive flow, without particulates in it, new pipe system components typically corrode very rapidly until a brittle scale develops on the surfaces exposed to the fluid. After this scale has developed, it forms a barrier between the metal and the fluid that substantially reduces the penetration rate. In this case, very low-level erosion is also taking place simultaneously with corrosion. In highly erosive flows, in which corrosion is also occurring, the erosion process dominates, and scale is scoured from exposed surfaces before it can influence the penetration rate. Corrosion therefore contributes little to material penetration. At intermediate conditions erosion and corrosion mechanisms can interact. In this case, scale can form and then be periodically removed by the erosive particles.
Droplet erosion occurs in wet gas or multiphase flow systems in which droplets can form. The erosion rate is dependent on a number of factors including the droplet size, impact velocity, impact frequency, and liquid and gas density and viscosity. It is very difficult to predict the rate of droplet erosion because most of these values are unknown in the field conditions.Experimental results indicate that under a wide range of conditions, the material lost by droplet erosion varies with time.
The hydrodynamics of the multiphase mixture within the pipeline also affects the degree of wetting of the pipe walls and the distribution of corrosion inhibitors injected into the pipeline system.
Above a certain velocity, the inhibitor film will be removed, leading to increased rates of corrosion. Currently, to determine the critical velocity, the following empirical correlations are used:
Cavitation ErosionWhen liquid passes through a restrictive low-pressure area, cavitation can be generated. If the pressure is reduced below the vaporization pressure of the liquid, bubbles are formed. These bubbles then collapse and generate shock waves. These shock waves can damage a pipe system. Cavitation is rare in oil and gas production systems because the normal operating pressure is generally much higher than the liquid vaporization pressures. Evidence of cavitation can be sometimes found in chokes, control valves, and pump impellers, but is unlikely to occur in other components.
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